An exemplary roof as shown in FIG. 1. consist of two primary components;
1) a roof covering 4 to shed water, most commonly asphalt/composite shingles, metal, tile or thatch material and,
2) a roof decking 3 to attach the covering to, most commonly ½ to ¾ inch thick plywood sheeting.
Roof rafters 2, serve to transmit the weight of the roof to the walls of the building, they are also used in the solar industry to structural attach a racking system to a roof, racking systems are generally a framework of two vertically separated rails for each row of panels that run horizontally across a roof, the solar panels or modules of an array are mechanically attached to the racking system and the racking system is structurally anchored to the rafters. Solar energy systems often seek to maximize their input one way this is done is by maximizing the area of the array and thus it is quite common for the array to be arranged in multiple rows, and cover most of a roofs area, this can require a great number of structural anchors. Rafters constructed from what is commonly referred to as 2×4s are widely used in the residential construction industry to provide structural support for a roof, and to also provide structural anchoring for anything that may be mounted to the roof. The geometry of a roof is such that the rafters of a roof run longitudinally from a roofs ridge, to the roofs edge and are spaced apart so that there longitudinal centers are commonly 16 inches apart, this is commonly referred to as having rafters on 16 inch centers, this center distance or spacing is not exact and it is quite common for some rafters to have more or less than the standard spacing. For a 24 ft span with 16 inch centers there will be 18 rafters, 16 will be intermediate meaning “in between” the first and the last rafter of the span, FIG. 1 shows a cut away view of a roof section with rafter's longitudinal centers 40. The problem is that these rafters or 2×4 s have a profile that is actually only 1½″×3½″ in length, with the 1½″ face supporting and underlying the roof decking and the 3½″ face perpendicular to the decking, the longitudinal centers of these rafters will also be perpendicular to the decking, these centers are not easily located from the outside of a roof which in the case of residential roofs is also commonly covered with shingles/composition roofing material. With the growing interest in residential roof mounted solar energy systems, which require the structural attachment of racking systems to the center of this underlying 1½″ face for the proper and approved mounting of solar systems, there is also a greater interest and need to provide a method, procedure and tool set for accurately locating these underlying rafter centers with precision and without damaging the roofing material in the process.
The structural attachment of a solar racking systems to underlying rafters will require multiple anchor points along parallel horizontal lines, vertically displaced from a roofs ridge, these structural attachments must meet or exceed a design pull-out strength requirement that is based on the anchor being centered in the structural member, A structural attachment attains its greatest pull-out strength when it is centered within the structural member, a loss of pull-out strength occurs for anchor point that are not within some variance of center, this variance is dependent on type of rafter, and type and size of anchor the National Design Specifications (NDS)—for wood construction, which are adopted in all model building codes in the U.S., requires an accuracy of “ 3/16ths of an inch” left or right of true center for a ¼″ lag bolt anchored into the 1½″ face of a 2×4, before loss of pull out strength occurs, it is therefore a necessary requirement that the greatest accuracy be used in finding these rafter centers as they will ultimately define the pull-out strength of any anchor used in any structural attachment.
The most common methods in use to date for finding these Rafter centers are; looking at the overhang of the roof if exposed, and measuring and projecting the course of the rafters from there endpoints, this method introduces a large percentage of error due to the one reference point at the end of each rafter and the fact that rafters may not run true for the desired length needed which leads to drilling test holes to make corrections, another method involves using expensive high-density stud finders which often require the removal of roof shingles or tiles to reduce the distance between the stud finder and the rafter and to provide a smooth surface for scanning, another method involves tapping the roof with a mallet or hammer, one can hear a noticeably different sound when rafter is struck, compared with hitting the space between the rafters this method is highly dependent on the thickness and condition of the roof and the skills of the user and may not always be repeatable or accurate, another method used from inside the attic involves measuring and recording the distance from the outside wall to the center of the first rafter, then directly measuring and recording center to center measurements of all rafters intermediate to the last rafter needed to support the racking system these measurements can then be transferred one measurement at a time to the roof using the outside wall as a starting point this method is highly dependent on the inside and outside measurement of the outside wall which is usually different do to siding or the fact that the outside wall does not extend above the roof an error in this measurement will be repeated at each subsequent measurement transferred to the roof, the direct measuring and transferring of individual center to center measurement is also error prone and this error will be stacked and additive such that each transferred center will likely contain a greater error.